Direct chill casting of ingots

ABSTRACT

In a direct chill casting of metal (e.g. aluminum) ingots, wherein an externally solidified ingot having an initially molten core is progressively withdrawn from a shallow, cooled, openended mold to which molten metal is progressively supplied, the ingot emerging from the mold passes successively through a first cooling zone extending from the mold for a predetermined distance along the path of ingot advance, and a second cooling zone located at that predetermined distance from the mold. Separate supplies of coolant fluid are respectively directed onto the ingot surface in the two zones, in such manner that the coefficient of heat transfer from the ingot to the coolant is substantially greater in the second zone than in the first. Specifically, the restricted intensity of cooling provided in the first zone is selected to maintain the outer portion of the ingot in solid state but preferably without completely solidifying the ingot core as the ingot traverses the first zone, while the greater intensity of cooling in the second zone effects complete solidification of the ingot core and simultaneously provides a high rate of cooling of the ingot periphery.

United States Patent [1 1 Bryson 1 1 3,713,479 1 Jan. 30, 1973 1541DIRECT CHILL CASTING OF INGOTS [75] Inventor: Neil Burton Bryson,

Ontario, Canada Kingston,

[73] Assignee: Alcan Research and Developmen t Ilimited, Montreal,Quebec, Canada [22] Filed: Jan. 27, I971 211 Appl. No.: 110,190

FOREIGN PATENTS OR APPLICATIONS 877,185 5/1953 Germany ..164/283 OTHERPUBLICATIONS Metal Industry, 10 October 1963. T8200. M586. Page 527.

Primary Examiner-R. Spencer Annear Attorney- Christopher C. Dunham, P.E. Henninger, Lester W. Clark, Robert S. Dunham, Gerald W. Griffin,Howard J. Churchill, R. Bradlee Boal, Robert Scobey and Henry T. Burke[57] ABSTRACT In a direct chill casting of metal (e.g. aluminum) ingots,wherein an externally solidified ingot having an initially molten coreis progressively withdrawn from a shallow, cooled, open-ended mold towhich molten metal is progressively supplied, the ingot emerging fromthe mold passes successively through a first cooling zone extending fromthe mold for a predetermined distance along the path of ingot advance,and a second cooling zone located at that predetermined distance fromthe mold. Separate supplies of coolant fluid are respectively directedonto the ingot surface in the two zones, in'such manner that thecoefficient of heat transfer from the ingot to the coolant issubstantially greater in the second zone than in the first.Specifically, the restricted intensity of cooling provided in the firstzone is selected to maintain the outer portion of the ingot in solidstate but preferably without completely solidifying the ingot core asthe ingot traverses the first zone, while the greater intensity ofcooling in the second zone effects complete solidification of the ingotcore and simultaneously provides a high rate of cooling of the ingotperiphery.

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DIRECT CHILL CASTING OF INGOTS BACKGROUND OF THE INVENTION Thisinvention relates to direct chill casting procedures for continuous orsemicontinuous production of metal ingots. In an important specificaspect, the invention is directed to procedures for direct chill castingof aluminum metal and alloys thereof, herein generically termedaluminum.

Although the procedures of the invention are broadly applicable to thecasting of any metal that can be satisfactorily cast in continuous orsemicontinuous manner by direct chill techniques, the invention will bedescribed herein for purposes of illustration with specific reference tothe continuous casting of aluminum ingots.

In a typical example of present-day practice, the continuous directchill casting of an aluminum ingot is effected in a shallow, open-ended,axially vertical mold which is initially closed at its lower end by adownwardly movable platform or stool. The mold is surrounded by acooling jacket through which a coolant fluid such as water iscontinuously circulated to provide external chilling of the mold wall.Molten aluminum is introduced to the upper end of the chilled mold, andas this molten metal solidifies in a region adjacent the periphery ofthe mold, the platform is moved downwardly. With effectively continuousdownward movement of the platform and correspondingly continuous supplyof molten aluminum to the mold, there is produced an ingot of desiredlength.

The ingot emerging from the lower end of the mold is externally solidbut is still molten in its central portion; in other words, the pool ofmolten aluminum within the mold extends downwardly into the centralportion of the downwardly moving ingot for some distance below the mold,as a sump of molten metal having a progressively decreasing crosssection as the ingot solidifies inwardly until its core portion becomescompletely solid.

As an important feature of the direct chill casting process,continuously supplied coolant fluid such as water is brought into directcontact with the outer surface of the advancing ingot below the mold.This direct chilling of the ingot surface serves both to maintain theperipheral portion of the ingot in solid state and to promote internalsolidification of the ingot.

Conventional direct chill casting procedure involves provision of asingle direct chill cooling zone below the mold. Typically, the coolingaction in this zone is effected by directing a substantial continuousflow of water (discharged, for example, from the lower end of the moldcooling jacket, and distributed substantially uniformly around theperiphery of the ingot) onto the ingot surface immediately below themold, just as the ingot emerges from the mold, in such manner that thewater impinges with considerable force on the ingot surface at asubstantial angle thereto, and flows downwardly over the ingot surfacewith continuing but diminishing cooling effect. Thus the greatestintensity of cooling is provided immediately below the mold outlet end,a locality which is ordinarily some distance above the level in the pathof ingot advance at which the ingot core solidifies completely.

At the described locality of greatest cooling intensity, the coefficientof heat transfer from the ingot to the cooling fluid is typically about0.5 cals./cm/sec./C, far

larger than the average value of the coefficient of heat transfer fromthe ingot to the mold (which is commonly about 0.05 cals./cm/sec./C),and also higher than the value of the ingot-to-coolant heat transfercoefficient at any lower level in the path of ingot advance. Typically,also, the thickness of the solidified ingot shell at this level ofmaximum cooling immediately below the mold is less than one fourth themaximum horizontal dimension of the ingot.

A problem encountered in conventional direct chill casting, especially(but not exclusively) in casting cylindrical ingots, is the tendency ofthe ingots to develop serious structural defects produced by socalledhot cracking, i.e., longitudinal center cracks formed incident tosolidification and cooling of the ingot. These defects render the ingotsunacceptable for many purposes. In the conventional practice describedabove, avoidance of hot cracking requires that the depth (verticalextent) of the molten metal sump below the lower edge of the mold bemaintained at a value not greater than the minimum transverse dimensionof the ingot, and indeed very comm only at a value no greater than twothirds of the minimum transverse dimension of the ingot.

For given cooling conditions, ingot dimensions and alloy composition,the sump depth is determined by the casting speed, i.e., the rate ofdownward advance of the ingot from the mold. Since conventional directchill casting systems are not designed to permit appreciable control ofcooling intensity, regulation of sump depth must be achieved byappropriate restriction of the casting speed. Specifically, theconstraint imposed on sump depth by the necessity of avoiding hotcracking has generally limited casting speeds to values between about 1and 7 inches per minute, depending on alloy composition and ingot sizeand shape. Such limitation of casting speed is undesirable from thestandpoint of productivity of the casting operation; it would be veryadvantageous, as enabling more rapid, efficient and economical'ingotproduction, to cast ingots at speeds substantially greater than thosenow attainable without producing hot cracking.

Efforts previously made to eliminate hot cracking have been predicatedon the assumption that hot cracking and cold cracking might be preventedin the same way; but expedients used to prevent cold cracking have notproven successful in preventing hot cracking, and have indeed sometimesincreased hot cracking susceptibility. In particular, it has heretoforebeen proposed to attempt to prevent hot cracking by reducing theintensity of cooling in all or part of the direct chill cooling zone, onthe theory that hot cracking was caused by residual tensile stresses inthe cast ingot, and that such stresses would be minimized if the ingotsurface were kept at a higher than normal temperature to reduce thetemperature differential between the ingot core and surface through andbeyond the region of core solidification. Expedients suggested for thispurpose have included reduction in volume and cooling efficiency of thesupplied direct-chill coolant, with use of a fog spray or pulsed watersupply to the ingot surface rather than a steady, impinging stream ofwater, or alternatively, removal of the cooling water from the ingotsurface after initial impingement on the surface, by a so-calledwipe-off operation. It has been found, however, that these expedients,with the possible exception of fog spray, do not enable any materialincrease in casting speed, i.e., without production of center cracking,and in at least some instances reduced cooling or wipe-off techniquesappear even to enhance the susceptibility of the ingots to hot cracking.

While the foregoing considerations have been discussed with reference toaluminum casting operations in which the ingot advances along a downwardvertical path, it will be understood that they are also applicable toother continuous or semicontinuous casting operations (such asoperations wherein the casting path does not have a verticalorientation), and in varying degree, to the direct chill casting ofmetals other than aluminum.

SUMMARY OF THE INVENTION The present invention broadly embraces thediscovery that advantageously superior freedom from center cracking indirect chill cast ingots, and very significantly enhanced casting speedsin direct chill casting operations, can be achieved by procedureincluding subjecting an ingot (advancing from a continuous casting mold)to the action of two separate direct chill cooling zones positioned insuccession along the path of ingot advance, wherein the first zone,extending from the mold for a predetermined distance along the path ofingot advance, provides a relatively restricted cooling intensity (asrepresented by magnitude of ingot-coolant heat transfer coefficient) andthe second zone, located at that predetermined distance from the mold,provides a substantially greater intensity of cooling. The direct chillcooling is effected by respectively directing separate supplies ofcoolant fluid (e.g., water) onto the ingot surface in the two zones, insuch manner as to provide the specified cooling intensities in the twozones. The first zone cooling is of such intensity as to maintain theperipheral portion of the ingot in solid state but preferably withoutcompletely solidifying the ingot core, while the cooling in the secondzone effects complete solidification of the core and at the same timeprovides a high rate of cooling of the peripheral portion of the ingot.Thus, in this procedure, the ingot encounters the greatest intensity ofcooling at or adjacent the locality (in its path of advance) at whichthe core becomes fully solidified, in contrast with present conventionalpractice, wherein the ingot encounters the greatest intensity of coolingimmediately beyond the outlet end of the mold, and ordinarily at leastsomewhat ahead of the locality of core solidification.

In the practice of the invention, a suitable range of positions for thelocality of application of the second zone coolant is that in which thedistance between the point of complete solidification of the ingot coreand the point of initial impingement of the second-zone coolant on theingot (as measured along the path of ingot advance) is not more thanabout one fourth the minimum transverse dimension of the ingot.Preferably, especially when a liquid such as water is used as the secondzone coolant, the locality of application of the second-zone coolant isahead of the point of core solidification, as it takes some time forwater or like coolant to begin to cool the ingot at the rate required atthe core solidification point. However, the locality of impingement ofsecond-zone coolant may be disposed beyond the point of coresolidification (i.e., within the stated range of positions) if thecoolant used extracts heat from the ingot sufficiently quickly so thatthe desired cooling rate can be achieved at the core solidificationpoint, ahead of the point of coolant application.

It is presently preferred, in use of a coolant such as water, to applythe coolant at a locality which is spaced ahead of the point of completecore solidification, along the path of ingot advance, by a distanceequal to about one sixth the minimum transverse dimension of the ingot.This may be contrasted with conventional direct chill castingoperations, utilizing a single direct chill cooling zone beyond themold, wherein the point of application of the direct chill coolant(water) is usually spaced ahead of the point of complete coresolidification by a distance (measured along the path of ingot advance)equal to about one half to two thirds the minimum transverse dimensionof the ingot.

Initial cooling of the ingot in the mold is performed in such manner asto maintain (within the mold) an average coefficient of heat transferfrom the ingot to the mold sufficient to produce a thin solid ingotshell at the mold outlet end having a thickness adequate to withstandfrictional stresses between the mold and the ingot. Further particularfeatures of the invention, in specific aspects thereof, reside in theprovision of an average coefficient of heat transfer from ingot tocoolant in the first zone equal to between about one and about six timesthe average heat transfer coefficient in the mold and preferably equalto at least about twice the average heat transfer coefficient in themold, and the provision of an ingot-coolant heat transfer coefficient inthe second zone equal to at least about one and one half times(preferably at least about five times) the average heat transfercoefficient in the first zone.

While broadly applicable to the casting of a wide variety of metals, theprocedures of the invention afford special advantages for the casting ofaluminum ingots, in overcoming the particularly serious center crackingproblems that have heretofore limited casting speed in production ofsuch ingots, and the invention in one specific sense is directedparticularly to aluminum casting procedures. In such procedures, theaverage coefficient of heat transfer from the aluminum to the mold istypically about 0.05 calories/cmlsecond/T; preferably, the averagecoefficient of heat transfer from the ingot surface to the coolantliquid in the first direct chill cooling zone is between about 0.1 andabout 0.2 caloriesIcm /secondPC; and also preferably, the coefficient ofheat transfer from the ingot surface to the coolant liquid in the secondzone is at least about 0.5 calories/cm /second/C.

it is found that the present invention enables production of sound,crack-free ingots even in casting operations wherein the sump depth(distance of the core solidification locality from the mold outlet end)is substantially greater than the minimum transverse dimension of theingot being cast; i.e., the invention overcomes the limitation as tosump depth heretofore considered essential for avoidance of centercracking. Thus the invention permits use of casting speeds far in excessof conventional ranges, with maintained freedom from hot cracking. Thisis so despite the fact that for any given set of casting conditions(ingot dimensions, alloy composition and casting speed), the sump depthin an ingot being cast by the present process will ordinarily besomewhat greater than that-in an ingot being cast by conventionalprocedure, owing to the reduced intensity of cooling (as compared withthe intensity of cooling encountered by the emerging ingot inconventional practice) in the first direct chill cooling zone.

In the present process as in prior practice, the sump depth (for givenconditions of ingot dimensions, alloy composition and coolingintensities) is directly-related to the casting speed; as the castingspeed increases, so does the sump depth. Accordingly, the distance fromthe mold to the second cooling zone is selected, with reference to thecontemplated casting speed, so as to position the second cooling zonewithin the abovedefined range of positions in relation to the localityin the path of ingot advance at which the sump terminates (i.e., thepoint at which the core of the ingot becomes completely solid), therebyto provide the desired increase in cooling intensity at that locality.Again in contrast to conventional casting operations, it is found thatthe procedure of the present invention affords superior flexibility ofoperation especially with respect to casting speed; if it is desired toincrease or decrease the casting speed, the second cooling zone ispositioned farther from or closer to the mold to accommodate thecorresponding change in sump depth, and the advantages of the inventionin preventing center cracking are again realized at the new castingspeed.

Without limitation of the invention by any particular theory, it is atpresent believed that the advantages of the invention are attributablein particular to the effect of the described direct chill cooling stepsand condi tions on the cooling rate of the peripheral portion of theingot at the locality (in the path of ingot advance) at which the ingotcore becomes completely solidified.

It is further believed at present that hot center cracking of ingots indirect chill casting operations is a consequence of excessive tensilestresses developed within the ingot at the locality at which coresolidification becomes complete. The tensile strength of the metal is ata minimum within a few degrees of the solidus point, and hence the coreimmediately after solidification is particularly susceptible to tensilestresses. Specifically, it is believed that crack-producing tensilestresses may be created at the locality of core solidification in thepath of ingot advance by an excessive disparity between the coolingrates (and hence the rates of contraction) of the ingot core andperiphery at that locality. The core metal, at the point ofsolidification, undergoes rapid cooling, and concomitantly rapidcontraction; if the cooling and contraction rates of the peripheralportion of the ingot at the critical locality are too low in relation tothe cooling and contraction rates of the core, center cracking(according to the present theory) results.

In conventional direct chill casting practice, with the greatestintensity of cooling applied immediately beyond the outlet end of themold and usually substantially ahead of the locality at which the coreof the advancing ingot solidifies, the peripheral portion of the ingotis very rapidly reduced in temperature as it emerges from the mold, andis thereafter cooled at a progressively diminishing rate as the ingotproceeds along its path of advance from the mold. Thus at the localityof core solidification, the cooling rate of the ingot periphery may bevery low in relation to the core cooling rate, especially as the castingspeed is increased, since increase in casting speed displaces thelocality of core solidification progressively farther from the localityof greatest cooling intensity. Accordingly, the present theory wouldindicate (as is in fact the case) that increase in casting speed, inconventional direct chill casting operations, enhances the likelihood ofcenter cracking. Reduced cooling and wipe-off techniques, heretoforeproposed for avoidance of center cracking, do not diminish and may evenaggravate the disparity between ingot core and peripheral cooling ratesat the locality of core solidification.

In the present procedure, in contrast, the cooling rate of theperipheral portion of the ingot is significantly higher (i.e., closer tothe cooling rate of the core) at the locality of complete solidificationof the core, than in conventional practice, for any given castingconditions and given casting speed. Owing to the relatively reducedcooling intensity in the first direct chill cooling zone of theinvention, the temperature of the ingot periphery remains relativelyhigh (in comparison to conventional practice) as the ingot approachesthe locality of complete core solidification. The maintained highperipheral temperature of the ingot facilitates attainment of a highperipheral cooling rate at the critical locality, since the cooling rateis dependent on the temperature differential between the ingot peripheryand the applied coolant; and this high cooling rate is then achieved bythe application of intense cooling in the second zone. Thus the directchill cooling steps and conditions of the invention cooperate to producea substantially higher ratio of ingot periphery cooling rate to ingotcore cooling rate at the locality of core solidification, resulting in acloser match between-core and peripheral contraction rates at thatlocality, than in conventional practice.

Further features and advantages of the invention will be apparent fromthe detailed description hereinbelow set forth, together with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic elevationalsectional view showing the practice of the present procedure in anillustrative embodiment;

FIG. 2 is an elevational sectional view of one specific form of directchill casting apparatus arranged to provide direct chill cooling in thefirst zone of the present procedure;

FIG. 3 is a similar view of an alternative form of apparatus forproviding the first zone direct chill cooling;

FIG. 4 is a graph in which the temperature of various points within analuminum ingot cast in accordance with the present procedure is plottedas a function of time of advance of such points beyond the casting mold;and

FIG; 5 is a graph similar to FIG. 2, showing the temperature ofcomparable points in an aluminum ingot cast by conventional procedureplotted as a function of time of advance of such points beyond thecasting mold.

DETAILED DESCRIPTION Referring to the drawings, FIG. 1 illustrates insimplified schematic view one form of apparatus for continuously castingaluminum ingots in accordance with the present invention. This apparatus(which is arranged for so-called vertical casting, i.e., for castingoperations wherein the ingot descends vertically from the mold as it iscast) includes an axially vertical annular mold 10 (open at its lowerend) to which molten aluminum metal 11 is supplied for casting an ingot12. The mold 10, fabricated of a metal suitably resistant todeterioration under conditions for casting aluminum, has a verticalinner wall 14 which defines an axially vertical casting zone 14a ofdesired horizontal cross section, it being understood that the mold wallconfiguration determines the cross-sectional shape of the producedingot; by way of specific illustration,

reference will be made herein to an annular mold wall 14 which iscylindrical, i.e., circular in cross section, for producing acylindrical ingot.

Surrounding the outer surface of the mold wall 14 is a cooling jacket15, shown for simplicity as formed of further wall portions cooperatingwith the wall 14 to define and enclose an annular chamber 15a completelylaterally surrounding the casting zone. This enclosed chamber issupplied as through a pipe 15b under control of a valve 15c with acooling fluid such as water for chilling the mold wall 14, and ispreferably kept continuously filled with a flowing or circulating bodyof the fluid, designated 16.

Located within the cooling chamber 15a is an annular baffle 17 disposedin concentric outwardly spaced relation to the mold wall 14 andextending vertically upward from the floor of the cooling chamber for adistance somewhat less than the height of the chamber. This baffledirects the circulating flow of water within the chamber 15a in suchmanner as to afford desired efficacy of cooling of the mold wall.

At the start of the casting operation, the lower end of the casting zone14a is closed by a stool or platform 18 which is supported on ahydraulic ram 20. As molten aluminum in the casting zone solidifiesaround the lower portion of the periphery of that zone, the stool 18 isdrawn vertically downward by operation of the ram 20. The solidifyingbase of the ingot being cast, resting on the stool, then begins toemerge from the lower end of the casting zone.

The mold apparatus is arranged to direct a spray of cooling fluid ontothe emerging solidified ingot surface immediately below the castingzone. Thus, as shown, an annular slit 22 (or a plurality of slits oropenings disposed in annular array) may be provided in the lower wall ofthe cooling jacket 15, extending entirely around the mold periphery andoriented to direct water from the chamber 15a of the cooling jacket ontothe surface of the emerging ingot with substantially uniformdistribution of water around the periphery'of the ingot. This spray ofwater, impinging on the ingot surface, acts to enhance the cooling ofthe ingot as it moves downwardly away from the mold. In the arrangementshown, water is continuously supplied to the chamber 15a and iscontinuously discharged through the slit 22 onto the ingot surface, sothat there is a continuous flow of coolant fluid for removing heat fromthe solidifying metal.

Molten aluminum metal is continuously supplied to the upper end of thecasting zone 14a as through a dip tube 24 that opens downwardly into theupper portion of the casting zone, so as to maintain the pool of moltenmetal in the casting zone at a substantially constant level as thesolidifying ingot is progressively withdrawn from the mold, i.e., as thestool is drawn downwardly.

During the described continuous casting operation, molten metal withinthe casting zone 14a solidifies around the periphery of the mold wall 14as it is cooled by heat transfer to the externally chilled mold surface.This solidification progresses sufficiently far inward toward the centerof the mold so that the ingot emerging from the lower end of the moldhas an externally solid and self-sustaining shell 25 even though thecentral portion or core 26 of the emerging ingot is still molten. Withan effectively continuous supply of molten metal to the mold, andcorrespondingly continuous downward advance of the cast ingot from themold, the molten central portion 26 of the ingot emerging from the moldextends downwardly as a molten metal sump (constituting the lower end ofthe molten metal pool in the mold) of progressively decreasing crosssection in a downward direction; in other words, aided by the effect ofthe cooling spray supplied through slit 22, the emerging ingotprogressively solidifies toward its center until, at a level 27 locatedat some distance below the lower end of the mold, the core of the ingotbecomes entirely solid.

The structures and procedures thus far described are, broadly speaking,conventional in present-day commercial direct chill continuous castingof aluminum ingots. Important features of the present invention residein the provision of special direct chill cooling conditions below themold, and more particularly in the provision of two direct chill coolingregions successively traversed by the descending ingot below the mold,as will now be explained.

The first of the direct chill cooling zones provided in accordance withthe invention extends from the outlet end of the mold for apredetermined distance along the path of advance of the ingot below themold, and is designated 28. In this zone, a first supply of coolantfluid, e.g., water, is directed onto the surface of the ingot forcooling the ingot, in a manner providing an average coefficient of heattransfer from the ingot to the coolant fluid in the first zone having avalue effective to maintain the ingot shell 25 in solid state whilemaintaining the core 26 of the ingot in molten state throughout thefirst zone.

The second direct chill cooling zone, designated 30, is located at thelast-mentioned predetermined distance from the mold outlet end. In thissecond zone, a second supply of coolant fluid (e.g., water), separatefrom the coolant fluid supplied in the first zone 28, is directed ontothe ingot surface for cooling the ingot in a manner providing acoefircient of heat transfer from the ingot surface to the coolant fluidin the second zone substantially greater than the average coefficient ofheat transfer in the first zone and effective to produce completesolidification of the ingot.

In the arrangement shown in FIG. 1, the supply of coolant fluid in thefirst direct chill zone 28 is the stream of water discharged onto theingot surface from the cooling jacket 15 through the slit or slits 22.In a conventional direct chill mold for vertical casting, the slit 22 isoriented to direct the spray against the ingot surface at a substantialangle (e.g., 30 to 45) to the vertical, so that despite vaporization ofthe water (effected by heat from the ingot shell surface) water inliquid state and in substantial volume comes into direct contact withthe ingot surface immediately below the ingot mold. In accordance withthe embodiment of the invention illustrated in FIG. 1, however, the slit22 is oriented to direct a spray of water against the ingot at asubstantially flatter angle (e.g., an angle of about to the vertical),and the volume of water thus discharged is also substantially reduced,as compared with conventional practice. For example, the volume of waterdischarged through slit 22 may be approximately half that conventionallyso discharged in the casting of an ingot of given size, configurationand composition; this reduction in volume of discharged water may beaccomplished by appropriate control of volume of water introduced to thecooling jacket.

In comparison with conventional vertical casting practice, the reducedangle of impingement of the water spray from slit 22, together with thereduced volume of sprayed water, greatly decreases the contact of liquidwater with the ingot surface immediately below the mold, especiallysince at the flattened angle of impingement of the spray there is agreater extent of vaporization of the water by the heat of the ingotbefore the water can reach the ingot surface. Consequently, throughoutthe first direct chill cooling zone, the cooling intensity (i.e., theaverage coefficient of heat transfer from the ingot surface to theapplied coolant over the extent of the first cooling zone) is verymaterially less than the cooling intensity immediately below the mold inconventional direct chill casting operations. It will be understood thatthis effect of reduction in impingement angle is found to occur invertical casting operations, but in horizontal casting (wherein theingot being formed advances along a horizontal path from the mold) asmall angle of impingement tends to increase cooling efficiency. Thus,in use of the invention in horizontal casting operations, the first-zonecooling is controlled in other ways, e.g., such as the alternativeshereinafter described with reference to FIGS. 2 and 3.

To provide the second direct chill cooling zone of the invention, in theembodiment of FIG. 1, a sub-mold cooling jacket or water ring 32 ispositioned at the lower end of the first cooling zone 28, in surroundingrelation to the descending ingot. The water ring 32 comprises an annularenclosed chamber, to which water is continuously supplied through aninlet pipe 34 controlled by a valve 35, and surrounds the ingotconcentrically. The inner wall 36 of the ring 32 is spaced outwardlyfrom the surface of the ingot for a sufficient distance to provideclearance for descent of the stool l8 and the ingot.

A vertically spaced annular slit 37 (or annular array of openings),surrounding the entire ingot and communicating with the interior of thewater ring 32, is provided in the ring inner wall 36 for directing astream of water from the ring onto the ingot surface. The slit 37 isoriented to direct the last-mentioned stream of water onto the ingotsurface at a substantially greater angle to the vertical than the spraydirected by the slit 22 described above; for example, the slit 37 may beoriented to direct water onto the ingot surface at an angle of 30 to 45to the vertical, corresponding to the angle at which water is usuallydirected onto an ingot surface from the lower end of a direct chillcasting mold in conventional casting operations as heretofore practiced.In addition, the volume of water directed through the slit 37 to theingot surface (determined by the volume of water supplied through pipe34 to the ring) is substantially greater than that discharged throughslit 22, being (for example) approximately equal to the volume of watercustomarily directed onto an ingot surface at the lower end of the moldin a conventional direct chill casting operation. Owing to the fact thatwater is discharged from the ring 32 onto the ingot surface in greatervolume and at a greater angle of impingement than the water dischargedthrough slit 22, liquid water in substantial volume comes into directcontact with the ingot surface in the second cooling zone 30. Thus, inthe second cooling zone, there is provided a substantially greatercooling intensity (coefficient of heat transfer from the ingot surfaceto the applied coolant) than in the first zone 28.

As will now be understood, the practice of the present method in theapparatus of FIG. 1, for continuously casting an aluminum ingot,includes the steps of supplying molten aluminum through the dip tube 24to the inlet end of the mold 10, while cooling the mold with water inthe cooling jacket 15) for solidifying the peripheral portion of thealuminum therein to form an ingot 12 having an externally solid shell 25and an initially molten core 26, and while continuously advancing theingot through and beyond the outlet end of the mold (by effectingcontinuous downward movement of the stool l8), and while cooling theingot beyond the mold for progressively solidifying the molten core toproduce a completely solidified ingot, the molten core extending as amolten metal sump within the ingot for a predetermined distance beyondthe mold outlet end along the path of advance of the ingot.Specifically, in the method of the invention, the ingot-cooling stepcomprises successively advancing the ingot through the first and seconddirect chill cooling zones defined above while providing in theserespective zones the above-described cooling conditions. This procedureis continued until an ingot of desired length has been cast.

As stated, the cooling conditions provided in the regions successivelytraversed by the descending ingot constitute especially importantfeatures of the invention. Initial cooling of the supplied molten metaloccurs within the mold, wherein the cooling conditions are such as toprovide an average coefficient of heat transfer from the metal to themold sufficient to produce at the mold outlet end a thin solid ingotshell having a thickness adequate to withstand frictional stressesbetween the mold and the ingot. In the method of the invention, thecooling conditions within the mold may be essentially comparable tothose heretofore conventionally employed and may, for example, providean average coefficient of heat transfer from the aluminum to the mold ofabout 0.05 calories/cmlsecondPC.

The average coefficient of heat transfer from the ingot to the coolantfluid in the first direct chill cooling zone 28, over the extent of thatzone, is maintained (by appropriate control of the volume and/or mannerof supply of coolant fluid) equal to between about one and about sixtimes the value of the aforementioned average coefficient of heattransfer in the mold, and is preferably equal to at least about twotimes the value of the average coefficient of heat transfer in the mold.A presently preferred range of values for the average coefficient ofheat transfer from the ingot surface to coolant liquid in the first zone28, for casting of aluminum ingots, is between about 0.1 and about 0.2calories/cm /second/C. As stated, the cooling intensity thus provided inthe first zone is such as to maintain the ingot shell 25 in solid stateand to solidify the major portion of the ingot cross section whilemaintaining at least the central core portion of the ingot in moltenstate throughout the extent of the first zone.

In the second direct chill cooling zone 30, the coefficient of heattransfer from the ingot surface to the coolant fluid is maintained(again by appropriate control of the volume and/or manner of supply ofcoolant fluid to the ingot surface in the zone 30) equal to at leastabout one and one half times the value of the aforementioned averagecoefficient of heat transfer in the first zone 28, and preferably equalto at least about five times the value of the average coefficient ofheat transfer in the first zone. In presently preferred practice for thecasting of aluminum ingots, the coefficient of heat transfer from theingot surface to coolant liquid in the second zone 30 is at least about0.5 calories/cm /second/C, i.e., about equal to or greater than theingot-coolant heat transfer coefficient provided immediately below themold in conventional operations for direct chill casting of aluminumingots. The second zone cooling intensity is, as stated, effective toproduce complete solidification of the ingot, the second zone having avertical extent (in the embodiment of FIG. 1) sufficient to effect suchcomplete solidification, i.e., to solidify the central core that remainsmolten throughout the first zone.

As already indicated, in the process of the invention the second directchill cooling zone 30 is positioned adjacent the level 27 at which thecore of the advancing ingot becomes completely solidified. Specifically,the second zone coolant water from slit 37 impinges on the surface ofthe descending ingot at a level which (in the illustrated embodiment ofthe invention) is spaced above the level of the sump extremity 27 by adistance equal to about one sixth the minimum transverse dimension (inthis case, the diameter) of the ingot.

Stated more generally, the point of impingement of the second zonecoolant on the ingot surface should be spaced from the extremity of themolten metal sump by a distance (along the path of ingot advance) equalto not more than about one fourth the minimum transverse dimension ofthe ingot. In use of a liquid such as water to provide the second zonecoolant, it is preferred that the second zone coolant impinge on theingot ahead of the sump extremity, and it is specifically preferred thatthe spacing between this locality of impingement and the sump extremityalong the path of ingot advance be equal to about one sixth the minimumtransverse dimension of the ingot, for attainment of the desired coolingeffect at the locality of complete solidification of the ingot core. Thepredetermined distance for which the first direct chill cooling zone 28extends below the mold is that between the lower end of the mold and thelevel at which the second zone coolant impinges on the ingot. Thus theingot encounters a relatively abrupt increase in cooling intensity(i.e., incident to passing from the first cooling zone 28 to the secondcooling zone 30) at or adjacent the level at which the core becomescompletely solid.

For an ingot of given dimensions and compositions, the depth of themolten sump 26 below the mold is dependent on the cooling conditionsencountered by the advancing ingot and on the speed of ingot advance.Owing to the reduced intensity of cooling in the first zone 28 (ascompared with the cooling intensity encountered by the ingot immediatelybelow the mold in conventional practice), the sump depth is greater forany given casting speed in the procedure of the present invention thanin conventional direct chill casting operations. The casting speed(i.e., rate of ingot advance from the mold) may also be greater in thepresent procedure than has heretofore been possible, providing stillfurther increase in sump depth.

In thearrangement of FIG. 1, the water ring 32, which constitutes thelower terminus of the first cooling zone and provides the second coolingzone, is positioned adjacent the level in the path of ingot advance atwhich the core becomes completely solid, as already explained. As willbe understood from the foregoing discussion, this position of the waterring is determined inter alia by the desired casting speed. Withincrease in casting speed, and concomitant increase in sump depth, thewater ring is positioned further below the mold so as to maintain thedesired positional relationship between the second cooling zone and thelower end of the sump (level 27).

To summarize, comparing the procedure of the present invention withconventional practice, it will be noted that the cooling conditionswithin the mold may be essentially the same as in present-dayconventional operations. However, the cooling intensity in the firstdirect chill zone (encountered by the ingot as it emerges from the mold)is very substantially lower than the intensity of cooling encountered bythe emerging ingot in conventional practice. The intensity of coolingapplied to the ingot in the second zone, at the predetermined distancefrom the mold adjacent the locality at which the core becomes completelysolidified, may be comparable to (or greater than) the cooling intensityencountered by the ingot as it emerges from the mold in conventionalpractice; but this region of most intense cooling is, as stated, spacedaway from the mold by the aforementioned predetermined distance, ratherthan (as in conventional operations) being located immediately adjacentthe mold and substantially ahead of I the locality of complete coresolidification.

The procedure of the present invention enables production of sound,crack-free ingots at casting speeds very substantially greater thanthose attainable in prior practice without center cracking. Thisadvantage is believed attributable to the fact that the presentprocedure provides a substantially higher cooling rate of the ingotperiphery, at the level of core solidification, than is achieved inconventional operations. The high peripheral cooling rate at such levelis attained by the application of a high intensity of cooling in thesecond cooling zone, and also by the relatively low intensity of coolingin the first cooling zone, which maintains the ingot periphery at acomparatively high temperature so as to enhance the rate of cooling ofthe periphery achieved by the cooling in the second zone. Provision of ahigh peripheral cooling rate at the level of core solidification reducesthe difference in contraction rates between the strongly cooling coreand the ingot periphery at such level and hence minimizes the tensilestresses between the core and the periphery which (as is now believed)have heretofore tended to cause center cracking in direct chill castaluminum ingots. In particular, the casting speed in the presentprocedure is not limited by the conventional requirement that the sumpdepth below the outlet end of the mold be no greater than the minimumtransverse dimension of the ingot, for avoidance of center cracking.

While the above-described use of reduced volume of water directed ontothe ingot at a flattened angle of impingement (as compared withconventional direct chill casting practice) represents one convenientway of providing the desired relatively low intensity cooling in thefirst direct chill cooling zone, such cooling may be accomplished inother ways e.g., similarly providing significant reduction in contact ofthe ingot surface by liquid water immediately below the mold. Forexample, a fog spray or mist comprising droplets of water entrained in aflow of air or other gas may be directed onto the ingot surface belowthe mold to provide the first zone cooling, or a pulsed (i.e.,intermittent) fiow of water may be directed onto the ingot surface atthat locality, in place of the conventional continuous flow.

Referring now to FIG. 2, there is shown an alternative form of moldconstruction arranged to provide the relatively low-intensity first zonedirect chill cooling in the process of the present invention. Thisapparatus includes an axially vertical annular mold wall 40 to which issecured structure defining a water box or cooling jacket 42 externallysurrounding the mold wall. Water supplied by suitable means (not shown)in continuous flow to the jacket 42 chills the mold wall to effectcooling of molten aluminum contained within the mold.

Mounted within the jacket 42 is an annular baffle 44, concentricallysurrounding the mold wall in adjacent but outwardly spaced relation tothe external surface of the wall and projecting upwardly from the floorof the jacket to a locality adjacent but somewhat below the top of thejacket. Water supplied to the jacket flows over the top of the baffleand down through the restricted annular space 45 defined between thebaffle and the mold wall to effect desired chilling of the mold. At itslower end, the space 45 opens into an annular chamber 46, from which thewater is discharged, in a manner hereinafter described, after descendingthrough the space 45.

Mounted within the chamber 46 is a ring 48, which concentricallysurrounds the mold wall 40 and cooperates with the floor of the coolingjacket to define a second annular chamber 50. The major flow of thewater entering chamber 46 from the space 45 passes over the uppersurface of the ring 48 and is discharged from the chamber 46 through aplurality of relatively large outlet passages 52. However, a minor flowof the water in chamber 46 enters the second chamber 50 through aplurality of holes 54 which are formed in the ring 48 and areindividually very substantially smaller in diameter than the outletpassages 52. Fluid from the chamber 50 is discharged downwardly towardthe surface of the ingot emerging from the mold through an annular slit56 (or an annular array of slits or openings) formed at the lower end ofthe mold wall; the slit 56 is oriented to conduct fluid from chamber 50and to direct it as a jet or spray at an acute angle to the emergingingot surface.

An annular air manifold 58 is mounted directly beneath the chamber 50and communicates therewith through a plurality of holes 60 equal indiameter to, and positioned in register with, the holes 54 which admitwater through ring 48 to the chamber 50. Air, forced into the manifoldby suitable means (not shown) through a plurality of passages 61, flowsupwardly into the chamber 50 through the holes 60, opposing the downwardflow of water into the chamber through the holes 54. This air mixes withthe water in chamber 50. The air-water mixture is expelled from thelatter chamber toward the ingot through the slit 56 as a fog or mist,comprising fine droplets of water entrained in the forced air flow;these droplets are vaporized by the heat of the ingot, providing a layerof steam around the ingot which affords some cooling of the ingot but ata substantially reduced cooling intensity as compared to a steady streamof water impinging on the ingot in liquid state. The cooling intensityin this embodiment can readily be controlled, to provide desired coolingconditions in the first direct chill cooling zone of the presentprocess, by adjustment of the supply of air to the manifold 58.

By way of example, in one illustrative structure embodying the featuresshown in FIG. 2, used with a mold having a vertical depth of 4 "/8inches for casting a 6- inch diameter aluminum ingot, the radialdimension of the space 45 was as inch. Six water outlet passages 52 wereprovided, spaced 60 apart around the circumference of the ingot, and sixair inlet passages 61 were similarly spaced 60 apart around the moldperiphery. The holes 54 and 60, each 1/16 inch in diameter, were spacedinch apart around the mold periphery. The annular slit 56 had a width ina range between 0.030 inch and 0.060 inch.

The described arrangement for providing the first zone direct chillcooling in the process of the invention may be substituted for the moldarrangement shown in FIG. 1. One advantage of the system of FIG. 2 isthe ease with which cooling conditions can be varied over a wide rangeof cooling intensities, even while a casting operation is in progress,by adjustment of air supply to the manifold, which changes theair-to-water ratio. Also, the mold may be used for conventional castingoperations, if desired, without structural modification; i.e., the moldwater may simply be discharged in continuous flow (without aeration)through the slit 56, in the same manner as in a conventional mold.

A further alternative arrangement of mold apparatus for providing thefirst zone cooling in the present process is illustrated in FIG. 3. Thisarrangement is adapted to provide a pulsed discharge of water from thelower end of the mold onto the surface of the emerging ingot. As in thestructures described above, the apparatus of FIG. 3 (which would replacethe mold l0 and cooling jacket 15 in the system of FIG. 1) includes anaxially vertical annular mold wall 64 adapted to receive molten aluminumfor continuous casting of an ingot, and structure providing a coolingjacket 66 laterally surrounding the mold wall. Water supplied bysuitable means (not shown) to the cooling jacket in continuous flowcirculates through the jacket and chills the mold wall externally. Alsoas in the abovedescribed mold structures, an annular baffle 68, mountedwithin the cooling jacket in spaced but closely surrounding relation tothe mold wall, defines an annular space 70 between the baffle and moldwall,.open at its upper end; the cooling jacket water flows over the topof the baffle and downwardly through the space 70, chilling the moldwall.

The inner surface 72 of the baffle 68 extends vertically downward fromthe upper end of space 70, in facing parallel relation to the outersurface of the mold wall. In the lower portion of the cooling jacket,the baffle surface 72 slopes downwardly and inwardly toward the moldwall as indicated at 73, then again extends vertically downwardly asindicated at 74, and finally slopes downwardly and outwardly away fromthe mold wall as indicated at 75 to the lower extremity of the coolingjacket. This surface 72 75 provides the normal path for flow of waterthrough and beyond the space 70 and owing to the outward slope of thelowermost portion 75 of the described surface, water following suchnormal path is discharged from the space 70 in a direction away from thesurface of the ingot 12.

The outer surface 77 of the mold wall 64 extends vertically downward toa level slightly below the inwardly sloping portion 73 of the bafflesurface 72. As will be seen from FIG. 3, the portion of space 70 definedbetween the lowermost extent of this vertical portion of the mold outersurface and the portion 74 of the mold inner surface is restricted inwidth (as compared to the upper portion of space 70) owing to the inwardslope of the baffle surface at 73.

At a locality opposite the baffle surface portion 74, a shoulder 78 isformed in the mold wall surface 77, and a further short vertical portion80 of surface 77, offset inwardly with respect to the major extent ofsurface 77, extends downwardly from the shoulder 78. Below surfaceportion 80, the mold outer surface slopes downwardly and inwardly towardthe ingot, as indicated at 81, to the lower end of the mold.

Means are provided in the structure of FIG. 3 for controllably divertingthe flow of water from the baffle surface portion 75 to the mold wallsurface portion 81, which directs the water inwardly toward and againstthe ingot surface. Specifically, outwardly of the baffle surface portion74 there is provided an annular chamber 83, concentrically surroundingthe mold wall and communicating with the lower portion of the space 70through plural axially horizontal holes 84 which open into the space 70,as shown, at a level slightly below the shoulder 78 of the mold wallsurface 77. These holes 84 are relatively small in diameter, e.g., ll 16inch.

A fluid such as water or air is supplied to the chamber 83 through meansillustrated schematically as a conduit 86 under control of a valve 87,which may be electrically operated and itself controlled by a suitabletiming device 88 for effecting intermittent supply of fluid to thechamber 83. When the valve 87 is open, the pressure developed in chamber83 by the supply of fluid through the conduit 86 forces the suppliedfluid through the holes 84. This secondary fluid flow reacts with themain flow of water descending through the space past the holes 84 insuch manner as to deflect that main flow of water against the mold wallsurface portion 81. Upon closing of the valve 87, the secondary fluidflow through holes 84 ceases, and the main water flow returns to thebaffle surface portion 75. The recess formed by shoulder 78 in the moldwall outer surface is vented to the atmosphere through small passages 90formed in the mold wall, in order to ensure that the main water flowwill not remain attached to the surface portion 81 (by the so-calledCoanda effect) after the deflecting force of the secondary fluid flow isshut off.

The operation of the apparatus of FIG. 3 to provide the desired firstzone cooling in the process of the present invention may now be readilyunderstood. With continuous supply of cooling water to the jacket 66 andcorrespondingly continuous flow of the water downwardly through space 70and thence out of the lower end of the mold, the timing device 88 isoperated to cause regular intermittent opening and closing of the valve87 and thereby to cause regular, intermittent supply of fluid throughthe conduit 86 to the chamber 83. Accordingly, the flow of waterdescending past the holes 84 in the space 70 is intermittently subjectedto the deflecting action of the secondary fluid flow through the holes84. Each time that secondary flow is applied, the main water flow isdiverted to surface portion 81 and is thus directed against the surfaceof the ingot emerging from the mold. Each time the secondary flow isinterrupted, the main water flow returns to the surface portion and isdiverted away from the ingot. Thus the ingot is subjected to a pulsed orintermittent stream of water in the first cooling zone rather than to acontinuous stream. The frequency and duration of the pulses, and hencethe supply of water per unit time to the ingot surface (which determinescooling intensity) are readily controlled by means of the timing device88.

As one further example of the process of the invention, the casting moldmay be positioned immediately above a pit filled with water throughwhich the emerging ingot descends, the pit and mold being so arrangedthat the ingot enters the water as it emerges from the mold. The heat ofthe ingot vaporizes water to form a jacket or barrier of steamthatsurrounds the descending ingot and inhibits contact of liquid waterwith the ingot surface. Within the pit, and spaced below the outlet endof the mold, there is provided awater ring (generally similar to thering 32 of FIG. 1) which directs jets of water against the ingotsurface; these jets penetrate the steam barrier to provide directcontact of the ingot surface with liquid water. In such arrangement, thefirst cooling zone is the portion of the ingot path of advance betweenthe outlet end of the mold and the water ring; in this zone, the steambarrier provides a relatively low intensity of cooling. The secondcooling zone is provided by the water ring, which effects cooling ofhigher intensity by causing contact of the ingot surface with liquidwater. The relative positions of the first and second zones are asdefined above the with reference to FIG. 1. Also, the arrangement ofapparatus elements may be essentially as shown in FIG. 1, with the moldwater discharge slit 22 omitted, and with coolant to the ingot in thefirst direct chill zone at plural localities spaced along the zone, toinsure maintenance of a solid ingot shell throughout the zone. It willbe understood that reference herein to the step of directing a firstsupply of coolant onto the ingot in the first direct chill zone embracesoperations wherein that coolant supply is directed onto the ingot fromplural sources and/or at plural localities along the path of ingotadvance, within the first zone; and it will be further understood thatwhere the first zone coolant is supplied from plural sources and/or atplural localities, such supply is controlled to provide throughout thefirst zone the cooling conditions described above, i.e., the specifiedconditions of average heat transfer coefficient.

The effect of the present process in reducing the disparity in coolingrates between the core and periphery of an ingot at the locality of coresolidification is illustrated in FIGS. 4 and 5, which show graphicallythe temperature of various points spaced radially outward from the coreof an ingot, as a function of time of advance of such points from acasting mold, in ingots cast respectively by the present process and byconventional direct chill casting procedure. Both ingots were 6-inchdiameter cylindrical ingots cast from the aluminum alloy identified bythe Aluminum Association designation AA6063, at a casting speed of nineinches per minute. The ingot temperatures were measured by thermocouplesimplanted in the ingot in a common horizontal plane and at variousdistances from the core.

In each of FIGS. 4 and 5, curves A, B, C, D and E respectively representtemperatures measured by thermocouples respectively located in a commonhorizon- 'tal plane at distances of approximately inch, 1 inch, 1 /2inches, 2 inches and 3 inches from the outer surface of the ingot.Cooling rates at each of these different calities are compared, in eachof FIGS. 4 and 5, for the interval during which the temperature of thecore of the ingot (curve E) decreased from 650 to 600C, i.e., the rangeof temperatures just below the temperature at which the core solidifies.

As stated, the ingot of FIG. 4 was cast in accordance with the presentprocedure, utilizing an arrangement of the type shown in FIG. 1, withthe water ring 32 positioned three inches below the outlet end of themold, and with water discharged from the mold slit 22 at a rate of 6 kimperial gallons per minute and from the water ring at a rate of 35imperial gallons per minute. As shown in FIG. 4, when the core of thisingot (curve E) began to cool through the 650-600C temperature range,the temperature of the periphery of the ingot as measured by theoutermost thermocouple located about $4 inch from the ingot surface(curveA) was about 300C, and the temperature measured by the latterthermocouple decreased by 25 while the core was cooling through 50 from650 to 600C. Thus, the

ratio of peripheral cooling rate to core cooling rate during thelast-mentioned period of core cooling was about 0.5.

The ingot represented by FIG. 5 was, as stated, cast by conventionalprocedure utilizing a single direct chill cooling zone below the moldwith the maximum intensity of cooling applied immediately below the moldoutlet end and substantially above the locality of core solidification.In this case, when the core began to cool through the 650 600Ctemperature range (curve E), the periphery of the ingot (again asmeasured by a thermocouple located about 56 inch inwardly of the ingotsurface, and represented by curve A in FIG. 5) was at a temperaturebelow 250C; and while the ingot core cooled from 650 to 600C, the ingotperiphery cooled through only 10C. Hence, in this case the ratio ofingot periphery cooling rate to core cooling rate was 0.2.

In short, the present procedure greatly reduced the disparity in coolingrates between the core and periphery of an ingot at the locality atwhich the ingot core had just become completely solid and was coolingthrough the 650 600C temperature range. This result may be attributedboth to the heightened intensity of cooling at that locality provided bythe present invention and by the higher temperature of the ingotperiphery at the point of core solidification, attained by the presentinvention as a result of the reduced intensity of cooling in the firstdirect chill cooling zone. The conventionally cast ingot of FIG. 5exhibited severe center cracking, while the ingot of FIG. 4 cast by thepresent procedure was sound and crack-free.

By way of further illustration of the procedure of the presentinvention, reference may be had to the following specific examples ofcasting of 6-inch diameter cylindrical ingots of grain-refined AA6063alloy. In each example, the ingot was cast in an axially vertical moldcooled by circulation of water through a surrounding cooling jacket, andthe second direct chill cooling zone of the invention was provided by awater ring of the type schematically shown at 32 in FIG. 1, spaced belowthe mold.

EXAMPLE I An ingot was cast at a speed of nine inches per minute withflow of water through the mold cooling jacket at a rate of 20 imperialgallons per minute and a water flow rate of 15 imperial gallons perminute through the sub-mold water ring which was located 3 inches belowthe lower end of the mold. The first zone cooling was provided bydirecting pulsed streams of water (1 second on, 2 seconds off) onto theingot surface immediately below the mold. The ingot was found to be freeof center cracking, although it had isolated surface cracks.

Another ingot was cast by the same procedure except that the first zonecooling was provided by directing aerated water (approximately 5imperial gallons per minute of water, in mixture with air) onto theingot surface immediately below the mold, and the water ring was located3 5: inches below the lower end of the mold. The resultant ingot wasentirely crackfree.

EXAMPLE II ing provided by discharge of water from the mold at a rate of6 imperial gallons per minute through a slit oriented to direct thewater onto the ingot surface at an angle of to the vertical. The waterring providing the second zone cooling was located 4 inches below themold. The ingot was entirely free of cracks.

Another ingot free of cracks was cast at the same speed in the sameapparatus, with the water ring positioned 3 inches below the mold, andwith water flowing at a rate of 8 imperial gallons per minute throughthe slit providing the first zone cooling, and at a rate of 35 imperialgallons per minute through the water ring.

EXAMPLE Ill Using the apparatus of Example II, but with the water ringlocated 7 inches below the mold, an ingot was cast at a speed of 12inches per minute. The water for the first zone cooling was dischargedthrough the 10 slit at a rate of 10 imperial gallons per minute, and thewater from the water ring was discharged at a rate of 35 imperialgallons per minute. The ingot was entirely free of cracks.

In contrast with the foregoing examples, ingots of the same alloy,dimensions and configuration cast by conventional direct chillprocedures (i.e., utilizing a single direct chill cooling zone below themold, with maximum intensity of cooling immediately below the mold)exhibited severe center cracking at casting speeds of 9 and 12 inchesper minute, although crack-free ingots were produced in thisconventional procedure at a casting speed of 6 inches per minute.Similar ingots cast with pulsed water cooling and with wipe-off ofdirect chill coolant two inches below the mold, but without use of ahigh intensity second direct chill cooling zone, also exhibited severecenter cracking at a casting speed of 9 inches per minute.

It is to be understood that the invention is not limited to the featuresand embodiments hereinabove specifically set forth but may be carriedout in other ways without departure from its spirit.

I claim:

1. Procedure for continuously casting an ingot, including the steps ofa. supplying molten metal to the inlet end of a casting mold having anopen outlet end, while b. cooling the mold for solidifying theperipheral portion of the metal therein to form an ingot having anexternally solid shell and an initially molten core, and while c.continuously advancing said ingot through and beyond said outlet end ofsaid mold, and while d. cooling said ingot beyond said mold forprogressively solidifying said molten core to produce a completelysolidified ingot, said molten core extending as a molten metal sumpwithin said ingot to an extremity of the sump beyond said mold outletend along the path of advance of said ingot; wherein the improvementcomprises:

e. the ingot-cooling step comprising i. in a first cooling zoneextending from said mold outlet end for a predetermined distance alongthe path of advance of said ingot, directing a first supply of coolantfluid onto the surface of said ingot for cooling said ingot, in a mannerproviding an average coefficient of heat transfer from said ingot to thecoolant fluid in said first zone having a value effective to maintainsaid ingot shell in solid state while maintaining the core of said ingotin molten state substantially throughout said first zone; and in asecond cooling zone, located at said predetermined distance from saidmold outlet end, separately directing a second supply of coolant fluidonto the ingot surface for cooling said ingot, in a manner providing acoefficient of heat transfer from the ingot surface to the coolant fluidin said second zone substantially greater than said average coefficientof heat transfer in said first zone, and effective to produce completesolidification of said ingot, thereby to provide a substantial increasein heat transfer coefficient as aforesaid at about said predetermineddistance from said mold outlet end; and

f. the ingot-advancing step comprising advancing said ingot at a ratesuch that said molten sump terminates, by solidification of the centerof said ingot, at a locality adjacent said predetermined distance beyondsaid mold outlet end, for maintaining the extremity of said sump at alocality adjacent the locality of substantial increase in heat transfercoefficient as aforesaid.

2. Procedure according to claim 1, wherein said average coefficient ofheat transfer in said first zone is substantially higher than thecoefficient of heat transfer from said metal to said mold within saidmold.

3. Procedure according to claim 1, wherein said path of ingot advance isoriented vertically downward; and wherein the step of directing a firstsupply of coolant fluid onto the surface of said ingot comprisesdirecting a first continuous stream of liquid onto the ingot surface ata preselected acute angle of impingement and wherein the step ofdirecting a second supply of coolant fluid onto the ingot surfacecomprises directing a second continuous stream of liquid onto the ingotsurface at an angle of impingement substantially greater than the angleof impingement of said first stream.

4. Procedure according to claim 3, wherein the volume of liquid per unittime directed onto the ingot surface in said second zone issubstantially greater than the volume of liquid per unit time directedonto the ingot surface in said first zone.

5. Procedure according to claim 1, wherein the step of directing a firstsupply of coolant fluid onto the surface of said ingot for cooling saidingot comprises mixing a flow of gas with a flow of liquid forentraining the liquid in the gas and directing the mixture of gas andentrained liquid onto the ingot surface.

6. Procedure according to claim 1, wherein the step of directing a firstsupply of coolant fluid onto the surface of said ingot comprisesintermittently directing a stream of liquid onto the ingot surface.

7. Procedure according to claim 6, wherein the step of intermittentlydirecting a stream of liquid onto the ingot surface comprisesestablishing and maintaining a continuous flow of liquid in thedirection of advance of the ingot and alternately directing said flowagainst and away from the ingot surface.

8. Procedure according to claim 1, wherein the step of directing a firstsupply of coolant fluid onto the surface of said ingot comprisesestablishing and maintaining a body of liquid in surrounding relation tosaid ingot at least substantially throughout the extent of said flrstzone, said ingot advancing through said body of liquid in said firstzone, and heat from said ingot vaporizing liquid of said body adjacentthe surface of said ingot, and wherein the step of directing a secondsupply of coolant fluid onto the ingot surface comprises directing astream of said liquid against the ingot surface at a substantial angleof impingement in said second zone.

9. Procedure according to claim 1, wherein said molten metal isaluminum.

10. Procedure according to claim 9, wherein the average coefficient ofheat transfer from said aluminum to said mold within said mold is notmore than about 0.05 calories/cm /secondPC; wherein said averagecoefficient of heat transfer from said ingot surface to the coolantfluid in said first zone is between about 0.1 and about 0.2 calories/cm/second/C; and wherein said coefficient of heat transfer from saidingotsurface to the coolant fluid in said second zone isat least about 0.5calories/cm /second/C.

11. Procedure for continuously casting an ingot, including the steps ofa. supplying molten metal to the inlet end of a casting mold having anopen outlet end, while cooling the mold for solidifying the peripheralportion of the metal therein to form an ingot having an initially moltencore, the mold-cooling. step providing an average coefficient of heattransfer from the metal to the mold sufficient to produce at the moldoutlet end a thin solid ingot shell having a thickness adequate towithstand frictional stresses between the mold and the ingot, and while0. continuously advancing said ingot through and beyond said outlet endof said mold, and while d. cooling said ingot beyond said mold forprogressively solidifying said molten core to produce a completelysolidified ingot, said molten core extending as a molten metal sumpwithin said ingot to an extremity of thesump beyond said mold outlet endalong the path of advance of said ingot; wherein the improvementcomprises:

e. the ingot-cooling step comprising i. in a first cooling zoneextending from said mold outlet end for a predetermined distance alongthe path of advance of said ingot, directing a first supply of coolantfluid onto the surface of said ingot for cooling said ingot, in a mannerproviding an average coefficient of heat transfer from said ingot to thecoolant fluid in said first zone over said predetermined distance equalto between about one and about six times the value of said averagecoefficient of heat transfer in said mold, said average coefficient ofheat transfer in said first zone being sufficient to maintain said ingotshell in solid state and to solidify the major portion of the ingotcross section; and

. in a second cooling zone, located at said predetermined distance fromsaid mold outlet end, separately directing a second supply of coolantfluid onto the ingot surface for cooling said ingot, in a mannerproviding a coefficient of heat transfer from the ingot surface to thecoo lant fluid in said second zone equal to at least about one and onehalf times the value of said average coefiicient of heat transfer insaid first zone, said predetermined distance being of such value thatsaid second supply of coolant fluid impinges on the ingot surface at alocality adjacent the extremity of said molten metal sump within saidingot beyond said mold outlet end along the path of ingot advance, andsaid second zone extending along the path of advance of said ingot for adistance sufficient to effect complete solidification of said ingot.

12. Procedure according to claim 11, wherein said average coefficient ofheat transfer in said first zone is equal to at least about two timesthe value of the average coefficient of heat transfer in the mold.

13.. Procedure according to claim 12, wherein said coefficient of heattransfer in said second zone is equal to at least about five times thevalue of the average coefficient of heat transfer in said first zone.

14. In apparatus for continuously casting an ingot, in combination,

a. an annular mold adapted to receive and contain a supply of moltenmetal for casting into an ingot, said mold having an open outlet endarranged for advance of the ingot through and beyond said mold outletend along a defined path as the peripheral portion of said ingotsolidifies within said mold;

. a cooling jacket laterally surrounding the mold for receiving andconducting a continuous flow of coolant liquid for cooling the mold;

. an annular chamber surrounding the outlet end of the mold, saidchamber having plural apertures communicating with the jacket foradmission of liquid from the jacket to the chamber and said chamberfurther including passage-defining means for discharging fluid from thechamber toward the surface of said ingot beyond the mold outlet end;

d. means independent of said chamber for discharging liquid from saidcooling jacket away from contact with said ingot; and

. means for controllably supplying gas to said chamber for opposingintroduction of liquid to said chamber through said apertures and formixture with liquid within said chamber such that the fluid dischargedfrom said chamber through said passage-defining means toward said ingotcomprises a mixture of gas and liquid. I

15. In apparatus for continuously casting an ingot, in

combination,

a. an annular mold adapted to receive and contain a supply of moltenmetal for casting into an ingot, said mold having an open outlet endarranged for advance of the ingot through and beyond said mold outletend along a defined path as the peripheral portion of said ingotsolidifies within said mold;

. a cooling jacket laterally surrounding the mold for receiving andconducting a continuous flow of coolant liquid for cooling the mold;

. means defining a passage for conducting a stream of the liquid withinthev cooling jacket in a direction substantially parallel to thedirection of advance of said ingot, said passage having an outlet endopening in the direction of advance of said distance equal to not morethan about one fourth the ingot adjacent the mold outlet end, the outletend minimum transverse dimension of said ingot.

of said passage being defined by diverging surfaces 17. Procedureaccording to claim 16, wherein said respectively sloping toward and awayfrom said insecond supply of coolant fluid impinges on the ingot got,and said passage being further arranged s surface ahead of the sumpextremity in the path of adthat liquid of said stream follows and isdirected by Vance of Said g one of said divergent surfaces as the streamis A meihod according to claim 17, wherein said discharged through thepassage outlet end;and second supply of coolant fluid impinges on theingot d. means for controllably applying to the stream, ad- Surface at alocality spaced from the p extremiiy jacent the passage outlet end andin a direction along the P Ofingot advance by a distance equal totransverse to the path f fl f the Stream, a fl not more than about onesixth the minimum transverse of fluid effective to deflect liquid ofsaid stream dimension of Said ingotfrom said one diverging surface tothe other of said procedul'e according to claim 11, wherein thediverging surfaces. distance between said mold outlet end and said sump16. Procedure according to claim 11, wherein said extremity along P off? of Said ingot, is second supply of coolant fluid impinges against thesurf f f than mm'mum transverse face of said ingot at a locality spacedfrom the sump exdlmens'on of sald mgot' tremity along the path ofadvance of said ingot by a

1. Procedure for continuously casting an ingot, including the steps ofa. supplying molten metal to the inlet end of a casting mold having anopen outlet end, while b. cooling the mold for solidifying theperipheral portion of the metal therein to form an ingot having anexternally solid shell and an initially molten core, and while c.continuously advancing said ingot through and beyond said outlet end ofsaid mold, and while d. cooling said ingot beyond said mold forprogressively solidifying said molten core to produce a completelysolidified ingot, said molten core extending as a molten metal sumpwithin said ingot to an extremity of the sump beyond said mold outletend along the path of advance of said ingot; wherein the improvementcomprises: e. the ingot-cooling step comprising i. in a first coolingzone extending from said mold outlet end for a predetermined distancealong the path of advance of said ingot, directing a first supply ofcoolant fluid onto the surface of said ingot for cooling said ingot, ina manner providing an average coefficient of heat transfer from saidingot to the coolant fluid in said first zone having a value effectiveto maintain said ingot shell in solid state while maintaining the coreof said ingot in molten state substantially throughout said first zone;and ii. in a second cooling zone, located at said predetermined distancefrom said mold outlet end, separately directing a second supply ofcoolant fluid onto the ingot surface for cooling said ingot, in a mannerproviding a coefficient of heat transfer from the ingot surface to thecoolant fluid in said second zone substantially greater than saidaverage coefficient of heat transfer in said first zone, and effectiveto produce complete solidification of said ingot, thereby to provide asubstantial increase in heat transfer coefficient as aforesaid at aboutsaid predetermined distance from said mold outlet end; and f. theingot-advancing step comprising advancing said ingot at a rate such thatsaid molten sump terminates, by solidification of the center of saidingot, at a locality adjacent said predetermined dIstance beyond saidmold outlet end, for maintaining the extremity of said sump at alocality adjacent the locality of substantial increase in heat transfercoefficient as aforesaid.
 2. Procedure according to claim 1, whereinsaid average coefficient of heat transfer in said first zone issubstantially higher than the coefficient of heat transfer from saidmetal to said mold within said mold.
 3. Procedure according to claim 1,wherein said path of ingot advance is oriented vertically downward; andwherein the step of directing a first supply of coolant fluid onto thesurface of said ingot comprises directing a first continuous stream ofliquid onto the ingot surface at a preselected acute angle ofimpingement and wherein the step of directing a second supply of coolantfluid onto the ingot surface comprises directing a second continuousstream of liquid onto the ingot surface at an angle of impingementsubstantially greater than the angle of impingement of said firststream.
 4. Procedure according to claim 3, wherein the volume of liquidper unit time directed onto the ingot surface in said second zone issubstantially greater than the volume of liquid per unit time directedonto the ingot surface in said first zone.
 5. Procedure according toclaim 1, wherein the step of directing a first supply of coolant fluidonto the surface of said ingot for cooling said ingot comprises mixing aflow of gas with a flow of liquid for entraining the liquid in the gasand directing the mixture of gas and entrained liquid onto the ingotsurface.
 6. Procedure according to claim 1, wherein the step ofdirecting a first supply of coolant fluid onto the surface of said ingotcomprises intermittently directing a stream of liquid onto the ingotsurface.
 7. Procedure according to claim 6, wherein the step ofintermittently directing a stream of liquid onto the ingot surfacecomprises establishing and maintaining a continuous flow of liquid inthe direction of advance of the ingot and alternately directing saidflow against and away from the ingot surface.
 8. Procedure according toclaim 1, wherein the step of directing a first supply of coolant fluidonto the surface of said ingot comprises establishing and maintaining abody of liquid in surrounding relation to said ingot at leastsubstantially throughout the extent of said first zone, said ingotadvancing through said body of liquid in said first zone, and heat fromsaid ingot vaporizing liquid of said body adjacent the surface of saidingot, and wherein the step of directing a second supply of coolantfluid onto the ingot surface comprises directing a stream of said liquidagainst the ingot surface at a substantial angle of impingement in saidsecond zone.
 9. Procedure according to claim 1, wherein said moltenmetal is aluminum.
 10. Procedure according to claim 9, wherein theaverage coefficient of heat transfer from said aluminum to said moldwithin said mold is not more than about 0.05 calories/cm2/second/*C;wherein said average coefficient of heat transfer from said ingotsurface to the coolant fluid in said first zone is between about 0.1 andabout 0.2 calories/cm2/second/*C; and wherein said coefficient of heattransfer from said ingot surface to the coolant fluid in said secondzone is at least about 0.5 calories/cm2/second/*C.
 11. Procedure forcontinuously casting an ingot, including the steps of a. supplyingmolten metal to the inlet end of a casting mold having an open outletend, while b. cooling the mold for solidifying the peripheral portion ofthe metal therein to form an ingot having an initially molten core, themold-cooling step providing an average coefficient of heat transfer fromthe metal to the mold sufficient to produce at the mold outlet end athin solid ingot shell having a thickness adequate to withstandfrictional stresses between the mold and the ingot, and while c.continuously advancing said ingot through And beyond said outlet end ofsaid mold, and while d. cooling said ingot beyond said mold forprogressively solidifying said molten core to produce a completelysolidified ingot, said molten core extending as a molten metal sumpwithin said ingot to an extremity of the sump beyond said mold outletend along the path of advance of said ingot; wherein the improvementcomprises: e. the ingot-cooling step comprising i. in a first coolingzone extending from said mold outlet end for a predetermined distancealong the path of advance of said ingot, directing a first supply ofcoolant fluid onto the surface of said ingot for cooling said ingot, ina manner providing an average coefficient of heat transfer from saidingot to the coolant fluid in said first zone over said predetermineddistance equal to between about one and about six times the value ofsaid average coefficient of heat transfer in said mold, said averagecoefficient of heat transfer in said first zone being sufficient tomaintain said ingot shell in solid state and to solidify the majorportion of the ingot cross section; and ii. in a second cooling zone,located at said predetermined distance from said mold outlet end,separately directing a second supply of coolant fluid onto the ingotsurface for cooling said ingot, in a manner providing a coefficient ofheat transfer from the ingot surface to the coolant fluid in said secondzone equal to at least about one and one half times the value of saidaverage coefficient of heat transfer in said first zone, saidpredetermined distance being of such value that said second supply ofcoolant fluid impinges on the ingot surface at a locality adjacent theextremity of said molten metal sump within said ingot beyond said moldoutlet end along the path of ingot advance, and said second zoneextending along the path of advance of said ingot for a distancesufficient to effect complete solidification of said ingot. 12.Procedure according to claim 11, wherein said average coefficient ofheat transfer in said first zone is equal to at least about two timesthe value of the average coefficient of heat transfer in the mold. 13.Procedure according to claim 12, wherein said coefficient of heattransfer in said second zone is equal to at least about five times thevalue of the average coefficient of heat transfer in said first zone.14. In apparatus for continuously casting an ingot, in combination, a.an annular mold adapted to receive and contain a supply of molten metalfor casting into an ingot, said mold having an open outlet end arrangedfor advance of the ingot through and beyond said mold outlet end along adefined path as the peripheral portion of said ingot solidifies withinsaid mold; b. a cooling jacket laterally surrounding the mold forreceiving and conducting a continuous flow of coolant liquid for coolingthe mold; c. an annular chamber surrounding the outlet end of the mold,said chamber having plural apertures communicating with the jacket foradmission of liquid from the jacket to the chamber and said chamberfurther including passage-defining means for discharging fluid from thechamber toward the surface of said ingot beyond the mold outlet end; d.means independent of said chamber for discharging liquid from saidcooling jacket away from contact with said ingot; and e. means forcontrollably supplying gas to said chamber for opposing introduction ofliquid to said chamber through said apertures and for mixture withliquid within said chamber such that the fluid discharged from saidchamber through said passage-defining means toward said ingot comprisesa mixture of gas and liquid.
 15. In apparatus for continuously castingan ingot, in combination, a. an annular mold adapted to receive andcontain a supply of molten metal for casting into an ingot, said moldhaving an open outlet end arranged for advance of the ingot through andbeyond said mold outlet end along a defined path as the peripheralporTion of said ingot solidifies within said mold; b. a cooling jacketlaterally surrounding the mold for receiving and conducting a continuousflow of coolant liquid for cooling the mold; c. means defining a passagefor conducting a stream of the liquid within the cooling jacket in adirection substantially parallel to the direction of advance of saidingot, said passage having an outlet end opening in the direction ofadvance of said ingot adjacent the mold outlet end, the outlet end ofsaid passage being defined by diverging surfaces respectively slopingtoward and away from said ingot, and said passage being further arrangedso that liquid of said stream follows and is directed by one of saiddivergent surfaces as the stream is discharged through the passageoutlet end; and d. means for controllably applying to the stream,adjacent the passage outlet end and in a direction transverse to thepath of flow of the stream, a flow of fluid effective to deflect liquidof said stream from said one diverging surface to the other of saiddiverging surfaces.
 16. Procedure according to claim 11, wherein saidsecond supply of coolant fluid impinges against the surface of saidingot at a locality spaced from the sump extremity along the path ofadvance of said ingot by a distance equal to not more than about onefourth the minimum transverse dimension of said ingot.
 17. Procedureaccording to claim 16, wherein said second supply of coolant fluidimpinges on the ingot surface ahead of the sump extremity in the path ofadvance of said ingot.
 18. A method according to claim 17, wherein saidsecond supply of coolant fluid impinges on the ingot surface at alocality spaced from the sump extremity along the path of ingot advanceby a distance equal to not more than about one sixth the minimumtransverse dimension of said ingot.